The pieces of refractory material used for the continuous casting of molten metals, notably steel, are used to protect the stream of molten metal flowing from the ladle toward the distributor from direct contact with the air. These refractory pieces are subjected to a substantial thermal shock at the beginning of the casting, as well as erosion and chemical corrosion by the molten steel and the slag. They are then manufactured into materials capable of resisting this aggressive environment. The materials most used are fused silica and graphitized alumina or a composite material of graphitized alumina and graphitized zirconia. The first material is used basically because of this exceptional resistance to thermal shock, but it has limitations with regard to its erosion resistance for certain types of steel. The second material is much used because it permits longer casting times with aggressive steels.
Although the use of pieces of refractory materials, notably stream protection tubes, has considerably improved the quality of steels by avoiding direct contact of the molten metal stream with the air, a problem of air aspiration through the permeability of the material persists due to the aspiration effect that occurs in the runner under the effect of a rapid flow of the stream of liquid metal. This aspiration of air has the effect of oxidizing the molten metal and leading to the formation of alumina deposits in the runner. These deposits can range up to a complete plugging of the runner and an interruption of the casting. In addition, this air aspiration results in a degradation of the steel quality through the formation of inclusions of oxide and gas bubbles entrained in the stream. These bubbles induce needle-like hole in the billets, blooms or slabs. Another negative effect of air aspiration is to increase the nitrogen takeup in the steel between the ladle and the mold.
Attempts have been made to remedy the above shortcomings by reducing the permeability of refractory materials by the addition of groups of components that produce liquid phases at the operating temperature in order to stop up the pores of the refractory material. Nevertheless, this process has the disadvantage that the liquid phases diminish the hot properties of the refractory material, which results in a higher wear level of the refractory material by the steel and the slag. In view of the fact that attempts are currently being made to increase the service life of the refractory material to permit longer casting sequences, this solution is not acceptable.
The precise purpose of the invention is a coating material designed to be applied to a piece of refractory material for the continuous casting of steel that remedies the problem of the negative effect of air aspiration on the refractory material on which it is applied and, in particular, without reducing its resistance to erosion.
To this end, the invention concerns a coating applied to a piece of refractory material for the continuous casting of steel, characterized in that it is impermeable to air so as to reduce air aspiration due to permeability of the refractory.
The coating material is preferably comprised of viscous phases at the operating temperature of the piece of refractory material.